This invention generally relates to mechanical refrigeration or air conditioning systems having a fluid refrigerant pump, and more particularly to means of protecting the refrigerant pump from damage related to the loss of lubrication and cavitation as a result of vaporous refrigerant entering the pump.
Air conditioning and refrigeration systems generally comprise an evaporator, a condenser, an expansion device, and a compressor. I have previously improved the cooling capacity and efficiency of refrigeration and air conditioning systems by adding a liquid pump at the outlet of the receiver or condenser, as disclosed in my U.S. Pat. No. 4,599,873. The liquid pump boosts the pressure of the condensed refrigerant flowing into the expansion device by 5 p.s.i. or more, a process I call liquid pressure amplification. Liquid pressure amplification suppresses flash gas in refrigerant flowing to the expansion device, provides a uniform flow of liquid refrigerant thereto, substantially increasing cooling capacity and efficiency.
In air conditioning and refrigeration systems which incorporate liquid pressure amplification, sufficient vapor-free liquid refrigerant must be maintained at the inlet to the liquid refrigerant pump so as to prevent vapor from entering the pump. Vapor entering the pump can result in both loss of lubrication and the well-known pumping phenomenon of cavitation, which comprises the sudden collapse of vapor bubbles, resulting in the generation of shock waves which may quickly erode the pump.
Any liquid, including liquid refrigerant, begins to vaporize if the pressure of the liquid drops below its saturation pressure. A liquid is said to be at saturation when its vapor pressure equals its liquid pressure. Any subsequent drop in liquid pressure results in vaporization of the liquid since liquid pressure then exceeds the ambient pressure. Liquid refrigerant typically leaves the condenser at or near saturation. It can then fall below its saturation pressure as it flows through he pump suction line into the pump inlet and while being accelerated along a pump surface inside the pump.
Sound engineering design of systems which incorporate pumps includes providing for a net positive suction head (NPSH) at the pump inlet to minimize the likelihood of the occurrence of cavitation. NPSH refers to the amount by which the liquid pressure at the pump inlet exceeds the saturation pressure of the liquid. The need to provide for sufficient NPSH to prevent cavitation is an important requirement for a refrigeration or air conditioning system which includes a liquid refrigerant pump. The desired NPSH may be achieved by locating the liquid refrigerant pump inlet below the condenser or receiver, and therefore below the normal liquid level in the condenser or receiver. Under normal operating conditions, the resulting "column" of liquid refrigerant provides a NPSH corresponding to the density of liquid refrigerant and the vertical distance from the pump inlet to the liquid level in the receiver or condenser.
Despite a designer's best efforts, it is not possible to ensure that the pump inlet pressure will always meet or exceed the required minimum NPSH. During system operation for example, the liquid level in the condenser or receiver can fall sufficiently to cause the NPSH to fall below the required minimum. Space constraints often do not permit the desired vertical offset between the condenser or receiver and the liquid refrigerant pump inlet. In those instances, it has heretofore not been possible to protect the liquid refrigerant pump against cavitation or loss of lubrication resulting from low inlet pressure or loss of liquid refrigerant entering the pump. A need remains for a way to do so.
This need is met by the present invention, which will be more readily understood upon consideration of the following summary and detailed description, taken in conjunction with the accompanying drawings.